The sleep apnea syndrome afflicts an estimated 1% to 5% of the general population and is due to episodic upper airway obstruction during sleep. Those afflicted with sleep apnea experience sleep fragmentation and intermittent, complete or nearly complete cessation of ventilation during sleep with potentially severe degrees of oxyhemoglobin desaturation. These features may be translated clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other sequelae of sleep apnea include right ventricular dysfunction with cor pulmonale, carbon dioxide retention during wakefulness as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Continuous positive airway pressure (CPAP) devices have been developed to treat this disorder. CPAP devices deliver positive air pressure to the nasal passages of patients during sleep in order to maintain their airways in a continuously open state. Examples of sleep apnea treatments based on continuous positive airway pressure applied within the airway of the patient are disclosed in U.S. Pat. Nos. 4,655,213 and 5,117,819. Bi-level positive airway therapy for treatment of sleep apnea and related disorders is taught in U.S. Pat. No. 5,148,802. In bi-level therapy, pressure is applied alternately at relatively higher and lower prescription pressure levels. An improved apparatus for delivering pressurized air to the airway of a patient which includes a feedback/diagnostic system is described in U.S. patent application Ser. No. 08/378,467, the disclosure of which is hereby incorporated by reference. The above apparatus includes a feedback system having a sensor means which detects the patient's breathing patterns. Unlike other systems, the breathing pattern sensor is situated generally at the end of the breathing circuit remote from the patient.
A problem associated with positive airway pressure devices is a lack of moisture in the air delivered by these devices has a drying effect on patient airways which causes the patient to have considerable discomfort and difficulty sleeping.
Humidifiers have been developed for use with CPAP devices to humidify the air supplied to the patient. Currently-available passive humidifiers designed for this purpose typically have been little more than reservoirs partially filled with water through which the air circulates. The air passes over the water's meniscu and entrains whatever water vapor is available at or near the free water surface. The rate of evaporation is governed primarily by the internal energy of the water in the reservoir and the concentration gradient of water vapor in the direction normal to the free surface.
Because of the laminar nature in these prior devices the concentration gradient of water vapor normal to the free surface is relatively low. The average temperature of the water decreases with time because of the transfer of heat in the phase change at the surface. Hence, the internal energy of the water mass decreases with time, causing a decay in the rate of vaporization and, therefore, the humidity of the air delivered to the patient.
One humidifier of interest is disclosed in U.S. Pat. No. 4,807,616 comprises a humidifier body 100 including a heater 120 in the form of an electric 1 coil at the end of an insulating sleeve 122. The electrical coil is disposed so as to be totally immersed in the water within the body 100.
Another humidifier of interest is made by Respironics, Inc. of Murrysville, Pa., comprises a shallow water reservoir having an inlet and an outlet. Healthdyne Technologies produces a similar passover humidifier for use with CPAP systems which adds moisture to intake air as it flows over the water in the humidifier's channel maze.
A variation of the Respironics, Inc. humidifier is shown in U.S. Pat. No. 5,231,979 to Rose et al. The Rose patent discloses a humidifier having shelve and ledges disposed near the inlet and outlet for breaking up waves. A single baffle is used to create a U-shape. air pathway through the humidifier. In an embodiment which is not illustrated, a pair of spaced apart baffles may be used to create an essentially S-shaped or serpentine airflow path. Nevertheless, because the airflow enters, circulates through and exits the humidifier body substantially parallel to the water surface, the laminar and essentially unrestricted nature of the airflow suggests that such airflow could not create the air turbulence necessary to assure a robust rate of vaporization. The disclosed effect of the baffle is to produce a circulation in the air flow around the baffle, the "waves" of which flow are broken by shelves and ledges.
An advantage, therefore, exists for a humidifier in which waves are dissipated in advance of the outlet by baffles such that the shelves and ledges of Rose et al. are not needed. The simplistic design of Rose et al. requires these shelves and ledges to be directly beneath the outlet. In the present invention baffles and an accumulation chamber eliminate the need for such structure.
Another advantage exists, therefore, for a positive airway pressure device humidifier designed to increase localized air turbulence to yield a high vapor concentration gradient.
A further advantage exists for a total surface area of water in the humidifier which is significantly greater than that of current devices, further enhancing the rate of mass transfer.